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TECHNOLOGY PROGRAMMES

Satellite Power SystemsSolar energy used in space

BR-202

May 2003

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TECHNOLOGY PROGRAMMES

INTRODUCTIONThe success of a space mission is always linked to the

performance of technology. To have a technology ready when a

satellite flies, research and development must start years in

advance.This is the objective of the Technology Programmes of

the European Space Agency: to ensure effective preparation for

European space activities.

Solar cells are a good example of space technology. This

brochure gives you many examples of how they work on aspacecraft, and what challenges they must overcome.

I hope it will let you share the enthusiasm of the engineers who

wrote the stories. Above all, I hope it will help you to appreciate

the efforts of all those European engineers who work behind the

scenes on space projects, not only in the area of solar cells, but

also in many other fields. Without their hard work, Europes

success in space would not have been possible.

Niels E. Jensen

Head of the Technology Programmes Department

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Nothing can change its state or position without

energy. Just like many other machines, satellites

also need electrical power to function.

Solar powerEvery life form or machineneeds energy to function

When one is out in space, however, the problem is where to get that

power from. Here on the ground there are many ways to produce

electrical power - or rather to transform some other form of energy

into electricity. If you are at sea or on top of a mountain, for example,

you can use portable generators or batteries.

The Sun is a very powerful, clean and convenient source of power,

particularly for satellites. The only thing needed is a means to

convert the energy contained in the Suns radiation mainly light

and ultraviolet rays into electrical power.The most efficient way to

achieve this today is by using panels composed of semiconductor

photovoltaic cells.Solar panels, as they are usually called, are now

quite a common sight here on Earth, but they were first used in

space in 1958 to power the Vanguard satellite.

The Sun is

a very powerful,clean and convenient

source of power,

particularly for satellites

SATELLITE POWER SYSTEMS

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How do solar cells work?

Each one of the thousands photovoltaic cells to be

found in a solar panel is made of a semiconductor

material, mostly silicon, capable of converting the

light arriving from the Sun into an electrical current.

This is exactly the reverse of what happens in any of

the thousands light-emitting diodes (LEDs) to be

found on the front panels of almost all of

todays electronic equipment.

Semiconductors like silicon are rather strange materials.They are normally insulators, which means that an electric

current cannot pass through them, but it is possible to

change crystals made of these materials into conductors

by applying a sufficiently high voltage to them. What

happens is that the voltage applied across the material

pulls electrons orbiting the atoms of the crystal out of their

orbit, making them available to become part of an electric

current flowing through the material. This can be

interpreted as the voltage applied to the crystal making

the electrons jump the barrier that constrains them to orbit

around each atom.

In real photovoltaic cells, such as the Hubble Space

Telescope silicon solar cell shown here, the basic

materials, the doping and the shape of the junction are

chosen in such a way as to increase their capability of

transforming the light energy into electrical energy. Eachcell is capable of producing a small amount of current at

a relatively low voltage, more or less like a common

pen-light battery. Many of them have to be combined in

series to produce the amount of electric power needed

for a satellite to function and to meet the power

demands of its on-board instruments.

SEMICONDUCTOR

A silicon solar cell from the NASA-ESA Hubble Space

Telescope. Such cells have an operating efficiency of

about 14%

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ATERIAL

The capability of semiconductor crystalsallowing or not the flow of electric currents through them can be reinforced

by adding controlled quantities of properly chosen chemical elements.

The process, called doping, produces crystals with more (n-type) or

fewer (p-type) electrons available to be freed when a voltage is applied.

Two or more layers of semiconductors with different dopings (p/n

junctions) have slightly different conductivity characteristics.They are

used to build devices capable of controlling the current flow. Each

junction creates a barrier, similar to that present in the crystal itself,

but with the desired behaviour. The most fundamental characteristic

of a semiconductor p/n junction is that electrons can jump it very

easily only in one direction.When the junction is illuminated, a portion

of the lights energy is transferred to electrons in the materials, making

them jump the barrier as if a voltage had been applied to it. If there is

a circuit, maybe just a piece of metallic wire attached to the other end

of the crystals forming the junction, then the electrons will flow

through it, returning to where they started from. An electric current,

generated by what is a very basic photovoltaic cell, flows through the

circuit.

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Why are solar arrays

so large?Despite the strength of the Sun, the solar

arrays needed by an average-sized satellite

are quite large, due to the rather low

efficiency of the individual solar cells.

This is why most pictures of classical satellites show

a pair of long wings extending from their sides, which

are the solar panels.

More modern solar cells based on semiconductor

materials like gallium-arsenide/arsenium are now

becoming available, with efficiency figures nearly

double those of silicon cells.These new types of cells

will allow smaller solar arrays to be used on future

space missions.

When the Sun is far away?

A good example of a mission deep into space isESAs Rosetta project, which will rendezvous with

and land on a comet after travelling through space

for almost eleven years. In this case, the silicon

solar cells have to be specially designed to cope

with the very low temperatures and very low light

levels that Rosetta will have to endure during its

journey. Consequently, this spacecrafts solar

arrays remain impressively large.

SOLAR POWER

NUNANuna, a record-breaking solar-powered racing car

The power-system technologies developed for ESAs spacecraft have been

embodied in many exciting projects back on Earth in recent years.This has

included an entry in 2001 in the World Solar Challenge, a race for solar-

powered cars from Darwin to Adelaide in Australia, a distance of 3010 km.

The winner in 2001 at its first attempt was the Dutch car Nuna, which

exploited European space power-system knowhow and technologies.Thanks

to this unique support from ESA, the car established a new World Record

time of 32 hours 39 minutes.The team was led by an ESA astronaut, now

Head of the ESA Education Office, Wubbo Ockels.

The European space power-system technologies

used to give the Nuna car its winning edge

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What happens

when the Sun

is hidden?Solar power generation is very convenient in space,

especially because there are no clouds and the Sun never

sets. Or does it?

Satellites orbiting the Earth pass through a shadow region on the opposite

side of the Earth from the Sun. Depending on the type of orbit, this can

happen just a few times a year or every few hours. During these so-called

eclipses, the solar panels cannot produce electrical energy and the satellite

would not only be unable to operate, but would also freeze to incredibly low

temperatures (eventually around 270C) if a backup power source were not

available. Electrical energy therefore has to be stored onboard the spacecraft

when in sunlight for consumption during these eclipses.

There are essentially two ways of storing electrical energy that are used on

satellites, both of which rely on reversible chemical reactions. One is based on

cells very similar to those found in portable phones and other equipment with

rechargeable batteries.The other uses so-called fuel cells, a type of electrical

accumulator now being used experimentally in cars and buses.

The basic principle is the same. An electric current passing through the cell

causes a pair of substances to combine into a new chemical composite

transforming electrical energy into chemical energy. This charges the

accumulator, storing the electrical energy being delivered to it. When the

accumulator is then inserted into an electrical circuit containing a load, it will

produce a current, gradually releasing its charge: the chemical composite

splits back into its two components, generating a steady flow of electriccharges (an electric current) through the circuit.

Rechar gea ble ba tteries use solid substances that are easily packaged

into housings of various shapes. Car batteries also need water, very pure

water, since the substance they use to the store energy, namely sulphuric

acid (highly corrosive, which is why you should never open one of them!), has

to be dissolved for the battery to work. Unlike a battery, fuel cells generate

current rather than simply storing energy. This is achieved by combining

hydrogen and oxygen at a platinum membrane, with water as a by-product.

This water can also be accumulated in a tank and successively divided into

oxygen and hydrogen by electrolysis, i.e. by letting a current flow through it.

The International Space Station

(ISS) orbiting the Earth, in

December 2001 (photo NASA)

Using the energy

Using the energy produced by the solar panels or

retrieved from the accumulators requires the use ofsophisticated electronic devices, called power-

conditioning units. However, it is not possible for

most of the equipment aboard the satellite to be

directly connected to them.Many ESA satellites like

the Rosetta mission, therefore, utilise small devices

that guarantee optimal balance between the power

from the battery and the power from the solar

arrays, even under unfavourable conditions of

eclipse and low-incidence illumination. These very

advanced devices are called Maximum Power

Point Trackers.

International Space StationThe International Space Station has solar panels

comparable in span to the size of a football pitch in

order to generate an impressive 92 kW of power

the largest solar arrays in orbit to date

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What is space technologyall about? Why aresatell i tes as they are?

Satellites can generally be regarded as spacecraft that

receive signals and send them back to Earth.

However, these spacecraft are extremely complexand expensive each one costs millions of Euros

because they have to work and survive in space for

periods of up to 15 years.

To make this possible, a satellite has to produce its

own power, generating electricity from sunlight falling

on photovoltaic cells or solar panels. Batteries are

used to store the energy, so that the satellite can

continue to work when the Sun is eclipsed or far

away for example during a mission to visit a comet

or a distant planet.

The technology ofspace exploration

Satellites

Antennas are only one of the many

kinds of technology developed for

satellites and space missions.

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To position itself in space, a satellite has to

manoeuvre using its own small rocket engines. It also

has to maintain its orientation, using thrusters or

gyroscopes, otherwise it will tumble along its orbit

and its antenna will drift out of alignment with theEarth. Space is not a friendly environment either.

Satellites have to survive temperature variations of

more than 200C rather like someone standing in

front of a fireplace with a blazing fire while an air

conditioner pumps freezing air onto his back.Outside

the protection of the Earths atmosphere, the level of

radiation (UV, X-rays, gamma-rays and all sorts of

energetic particles) is much higher and more

destructive than on the ground. Before they can even

begin to operate in space, satellites have to survive

the bone-shaking launch.Then the solar panels have

to be opened and antennas, which are often stowed

to take less space in the launcher, deployed before

the satellite enters its operational orbit.

Once in orbit, a satellite usually carries out

multiple functions, with different payloads or

instruments. It then sends information to a ground

station about the condition of its payload and its

systems, and it receives instructions back from the

ground operators. All this has to work well, with little

possibility of recovery, not to mention repair. Apart

from very special cases, there is no way back.

Unlike other kinds of business, efficiency and

durability are not just advantageous but essential in

space.

This is why it takes years of work by many talentedengineers to design, build and check the correct

functioning of a satellite. This is what space

technology is all about.

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For further information contact:

Marco Sabbadini

Antenna Section

ESA Directorate of Technical and Operational Support

marco.sabbadini@esa.int

Giorgio Saccoccia

Head of Propulsion and Aerothermodynamics Division

ESA Directorate of Technical and Operational Support

giorgio.saccoccia@esa.int

Margherita Buoso

Coordinator of Communications

ESA Directorate of Industrial Matters

and Technology Programmes

margheri ta.buoso@esa.int

Published by:

ESA Publications Division

ESTEC, PO Box 299

2200 AG Noordwijk

The Netherlands

Editors:

Niels E. Jensen & Bruce Battrick

Design & Layout:

GPB,

Leiderdorp,

The Netherlands

Copyright:

ESA 2003

ISSN No: 0250-1589

ISBN No: 92-9092-794-1